Metal oxides, in particular mixed metal oxides, have a wide technical field of use such as for example in ceramics, polymer additives, fillers, pigments, reactive surfaces, catalysts, catalyst constituents, etc.
In particular, metal oxides are also used as catalysts, for example for the production of oxidic catalysts, in particular in the production of methanol or for the hydrogenation of esters. Important variables when using oxides as catalysts, or as catalyst precursors, or constituents thereof are in particular the metal surface area of the reduced metals.
This can be influenced by the composition of the educt(s) through the targeted control of the production process (crystallization process) of oxides. Here, an important factor is in particular the crystallite size in various catalyst systems, which have been, examined in detail by R. Schlögl et al. (Angewandte Chemie, 116, 1626-1637, 2004).
A further important parameter is the BET surface area of these particles, wherein, as a rule of thumb, it is the case that, the higher the BET surface area is, the higher the catalytic activity is also. There have therefore always been attempts to obtain a catalyst powder with a high BET surface area. Furthermore, small metal crystallites contribute to a large and active total metal surface area and thus promote a high catalytic activity.
So-called nanocrystalline powders have also increasingly been considered for this purpose, as these appear to be particularly suitable for catalytic applications. However, production problems, often unsolved, occur here. A nanocrystal is a crystalline substance the size of which lies in the nanometer range or, in other words, a nanoparticle with a very largely crystalline structure.
Nanocrystalline oxide powders were previously usually produced either by chemical synthesis (for example by coprecipitation, etc.), by mechanical methods or by so-called thermophysical methods. However, the desired BET surface area in this case achieves values in the range for example of copper oxides (e.g. by calcining Cu2OH2CO3) of only at most 60 to 90 m2/g.
Typically, in the chemical synthesis of nanocrystalline powders, starting from so-called precursor compounds, a powder is synthesized by chemical reactions, for example by means of hydroxide precipitation, synthesis by hydrolysis with organometallic compounds and hydrothermal methods. The final structure of the nanocrystallites as typically achieved only after or during the calcining of the thus-obtained products.
Mechanical production methods are disadvantageous, as inhomogeneous particle-size distributions are mostly achieved or the particles become amorphous because of the pressure exerted on the particles and also the general particle size is too large to obtain the desired small metal crystallites after the reduction.
Thermophysical methods, such as are described for example in WO 2004/005184, are also known. These are used in particular in the industrial-scale production of fine crystalline silicon dioxide in which readily volatile silicon compounds are sprayed into an oxyhydrogen flame.
So-called plasma synthesis methods are further known in which the starting materials are evaporated in a plasma hot to 6000 kelvin, or CVD methods in which gaseous products are reacted.
For the steam reforming of methanol, CuO/ZnO/Al2O3-based catalysts are usually used which often lack long-term stability and have a high carbon content in the catalyst. The product often has very high undesired levels of C in addition to hydrogen and carbon dioxide. The specific parameters of these systems have been examined in detail by H. Purnama, Catalytic Study of Copper based Catalysts for Steam Reforming of Methanol (Berlin doctoral thesis 2003).
The use in the hydrogenation of esters, for example of maleic acid dimethyl esters to 1,4-butanediol, γ-butyrolactone and tetrahydrofuran on CU/ZnO catalysts, typically produced by coprecipitation has been examined in detail by Schlander, Karlsruhe doctoral thesis 2000, and CuO-based catalysts with high activity have been described.
In particular, the intermediate product 1,4-butanediol with two terminal hydroxyl groups from which for example polybutylene terephthalate, polyurethane and polyester can be obtained is also attractive. In addition to the production of polymers, 1,4-butanediol is also further processed to tetrahydrofuran and γ-butyrolactone. In all of these products, there is an increased need and thus also the necessity to make available further more active and more selective catalysts.
Further details on copper-containing catalysts, in particular based on Cu/ZnO, are to be found in the doctoral thesis by C. Olinger, Untersuchung der einstufigen Gasphasenhydrierung von Dimethylmaleat zur Herstellung von γ-Butyrolacton, 1,4-Butandiol und Tetrahydrofuran (Karlsruhe doctoral thesis, 2005).
WO 2007/136488 A2 describes copper oxide nanoparticle systems with shell-like structure, wherein the core of a nanoparticle consists of either Cu2O or a mixture, not specified in more detail, of metallic Cu and Cu2O and CuO is found on parts of the surface of such a nanoparticle. The thickness of the layer of CuO is approximately 1 nm.
This shell-like catalyst system can be supported or unsupported. The Cu2O nanoparticles were produced as specified in WO 2005/060610, wherein the nanoparticles are stabilized during the synthesis by ligands such as oleic acid or trioctylamine.